This invention relates to a method and apparatus for measuring the distance between a metallic body, for example a steel stock, and a sensor of the measuring apparatus and more particularly to a method and apparatus of the type wherein a sensor, preferably a probe type sensor constituted by a resonance circuit including a coil and a capacitor, is used and the distance between the metallic body and the sensor is measured at high accuracies without contacting the metallic body.
As a method of measuring the distance between a metallic body and measuring sensor, a method utilizing the electromagnetic induction, as diagrammatically shown in FIG. 1 has been used. More particularly a coil 4 having an impedance Z.sub.2 is connected to form an AC bridge circuit 3 together with impedances Z.sub.1, Z.sub.3 and Z.sub.4 having known impedances. The input terminals of the AC bridge circuit 3 are connected across a reference oscillator 2 having a prescribed frequency and the output of the AC bridge 3 is amplified by a differential amplifier 5. When the distance D between the coil 4 and a metallic body 1 is substantially infinite, the AC bridge circuit 3 becomes balanced when a condition Z.sub.1.sup.. Z.sub.4 = Z.sub.2.sup.. Z.sub.3 is satisfied as is well known in the art. Under such balanced condition, the output from the AC bridge circuit 3 is zero and hence the differential amplifier 5 produces no output. As the distance D decreases the impedance Z.sub.2 of coil 4 varies with the distance D due to the variation of the self inductance caused by the electromagnetic induction in the metallic body. As is well known in the art the impedance Z.sub.2 varies non-linearly with respect to the distance D. As a result, the output of the bridge circuit 3 is also non-linear. This output is amplified by the differential amplifier 5 and then supplied to an indicating meter or a recording meter, thus measuring the distance D without contacting the metallic body 1. FIG. 2 shows one example of a characteristic between distance D and the output voltage V.sub.pp of the differential amplifier 5.
As shown in FIG. 2, the output characteristic of such prior art measuring apparatus is non-linear. Especially, as the distance D increases, the rate of varying the output decreases, so that high accuracies can not be attained. Accordingly, it is necessary to linearize the characteristic by using a suitable external circuit. Moreover, as the change in the impedance Z.sub.2 of the coil 4 with respect to the change in the distance D to the metallic body 1 is small, it is necessary to construct the bridge circuit 3 to be highly sensitive. The accuracy of the measurement is greatly influenced by the accuracy and characteristics of the fixed impedances Z.sub.1, Z.sub.3 and Z.sub.4 so that it requires a high degree of skill to adjust the bridge circuit.